The row with the London mayor, Boris Johnson, may have given Sir Ian Blair an opportunity to depart as commissioner of the Metropolitan police without going directly as a result of a scandal or controversy.

Controversy after controversy had left him weakened. But it was not scandals past that were the biggest danger to Blair, namely the De Menezes killing and the race row plaguing the Yard. It was the criticism to come over corruption allegations that left him weak to a point where the Tory mayor could pick him off.

It all added up to a situation where Blair's demise was increasingly not a matter of if, but when, he would become the first commissioner in modern times to be ousted from office.

Blair seemed to have more than an inkling of his demise back in July, when he warned the post of commissioner was becoming increasingly politicised. Speaking at a regular monthly press briefing, he said that plans by Johnson to take more control of the force could ultimately be damaging.

Blair said he was aware that Johnson was elected in May on a platform of wanting the power to hire and fire the commissioner, in order to make policing more accountable.

The commissioner said such a move could be a "poor bargain" for the Met, because it had national functions, in leading the police fight against terrorism, as well as an international role.

Blair said: "Most senior officers are concerned that the office of commissioner is becoming a matter for high politics, which it has been for quite some time."

His comments came immediately after it was revealed that a top Conservative close to Johnson had wanted to suspend the commissioner over allegations of cronyism and corruption.

The biggest and most consistent cloud over Blair for the past three years has been the shooting dead of Jean Charles de Menezes in July 2005. Blair has been criticised for his handling of the shooting, and making a bad situation worse.

But the biggest threat to Blair had become the contracts issue. He was under formal investigation over the awarding of £3m worth of police contracts to a close personal friend of his. Blair thought it was enough that he had openly declared his relationship with the businessman. But the commissioner's closest allies, including those who have publicly supported him, believed the contracts issue was toxic and potentially fatal.

It emerged that Blair had a sat on a panel that awarded the contract, and it was believed the official report would criticise the commissioner's judgment when completed either later this year, or early next. There was all this damage, despite even his biggest enemies accepting Blair had not personally benefited by a single penny.

Blair did face a hostile rightwing press determined to get him, but he also became bereft of supporters, in government and at the top of his own force. One senior police officer yesterday said: "The home secretary could have refused his resignation."

The Labour government stuck by him, in part because his commissionership did see crime fall, in part because they did not want to hand his scalp to their Tory opponents. Months after he started as commissioner, Blair talked to the Guardian about the challenges of the job: "You don't come into here without a pair of copper-bottomed trousers. This is a very tough place."

Recently he was heard to say he did not realize how "copper bottomed" his trousers would need to be. Nor must he have realised how much he would contribute to his own demise.